Two reasons mainly. First, PS4 is far outselling XB1 in just pure console units moved. The more popular console has the the easier time making a case for exclusivity. Second, have you ever hear the phrase "necessity is the mother of invention"? Well that applies to Sony here. Playstation and the gaming side is one of the only profitable divisions of Sony. Most of the others just bleed money. So it makes sense to invest in things that will continue to bring in profits. Paying third party developers for exclusives is part of that strategy.